[PATCH net 2/3] net: compound page support in skb_seq_read
From: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Date: 2021-01-08 17:13:25
Subsystem:
networking [general], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds
From: Willem de Bruijn <willemb@google.com>
skb_seq_read iterates over an skb, returning pointer and length of
the next data range with each call.
It relies on kmap_atomic to access highmem pages when needed.
An skb frag may be backed by a compound page, but kmap_atomic maps
only a single page. There are not enough kmap slots to always map all
pages concurrently.
Instead, if kmap_atomic is needed, iterate over each page.
As this increases the number of calls, avoid this unless needed.
The necessary condition is captured in skb_frag_must_loop.
I tried to make the change as obvious as possible. It should be easy
to verify that nothing changes if skb_frag_must_loop returns false.
Tested:
On an x86 platform with
CONFIG_HIGHMEM=y
CONFIG_DEBUG_KMAP_LOCAL_FORCE_MAP=y
CONFIG_NETFILTER_XT_MATCH_STRING=y
Run
ip link set dev lo mtu 1500
iptables -A OUTPUT -m string --string 'badstring' -algo bm -j ACCEPT
dd if=/dev/urandom of=in bs=1M count=20
nc -l -p 8000 > /dev/null &
nc -w 1 -q 0 localhost 8000 < in
Signed-off-by: Willem de Bruijn <willemb@google.com>
---
include/linux/skbuff.h | 1 +
net/core/skbuff.c | 28 +++++++++++++++++++++++-----
2 files changed, 24 insertions(+), 5 deletions(-)
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c858adfb5a82..68ffd3f115c1 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h@@ -1203,6 +1203,7 @@ struct skb_seq_state { struct sk_buff *root_skb; struct sk_buff *cur_skb; __u8 *frag_data; + __u16 frag_off; }; void skb_prepare_seq_read(struct sk_buff *skb, unsigned int from,
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index f62cae3f75d8..4acf45154b17 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c@@ -3442,6 +3442,7 @@ void skb_prepare_seq_read(struct sk_buff *skb, unsigned int from, st->root_skb = st->cur_skb = skb; st->frag_idx = st->stepped_offset = 0; st->frag_data = NULL; + st->frag_off = 0; } EXPORT_SYMBOL(skb_prepare_seq_read);
@@ -3496,14 +3497,27 @@ unsigned int skb_seq_read(unsigned int consumed, const u8 **data, st->stepped_offset += skb_headlen(st->cur_skb); while (st->frag_idx < skb_shinfo(st->cur_skb)->nr_frags) { + unsigned int pg_idx, pg_off, pg_sz; + frag = &skb_shinfo(st->cur_skb)->frags[st->frag_idx]; - block_limit = skb_frag_size(frag) + st->stepped_offset; + pg_idx = 0; + pg_off = skb_frag_off(frag); + pg_sz = skb_frag_size(frag); + + if (skb_frag_must_loop(skb_frag_page(frag))) { + pg_idx = (pg_off + st->frag_off) >> PAGE_SHIFT; + pg_off = offset_in_page(pg_off + st->frag_off); + pg_sz = min_t(unsigned int, pg_sz - st->frag_off, + PAGE_SIZE - pg_off); + } + + block_limit = pg_sz + st->stepped_offset; if (abs_offset < block_limit) { if (!st->frag_data) - st->frag_data = kmap_atomic(skb_frag_page(frag)); + st->frag_data = kmap_atomic(skb_frag_page(frag) + pg_idx); - *data = (u8 *) st->frag_data + skb_frag_off(frag) + + *data = (u8 *)st->frag_data + pg_off + (abs_offset - st->stepped_offset); return block_limit - abs_offset;
@@ -3514,8 +3528,12 @@ unsigned int skb_seq_read(unsigned int consumed, const u8 **data, st->frag_data = NULL; } - st->frag_idx++; - st->stepped_offset += skb_frag_size(frag); + st->stepped_offset += pg_sz; + st->frag_off += pg_sz; + if (st->frag_off == skb_frag_size(frag)) { + st->frag_off = 0; + st->frag_idx++; + } } if (st->frag_data) {
--
2.30.0.284.gd98b1dd5eaa7-goog